1. Technical Field
The present invention discloses a method for manufacturing a granulated inorganic adsorbent for radionuclides, and more particularly to a method for preparing a granulated inorganic adsorbent for radionuclides for removing 137Cs in a radioactive liquid waste at high selectivity.
2. Related Art
Nuclear is a mature power generation technology currently and is also one of energy technologies that can reduce carbon dioxide emissions, and today, a large number of counties around the world have put a lot of construction to develop the nuclear industry. Although nuclear power generation can bring people many convenience in life, in the application process, many radioactive or radioactively contaminated wastes are generated, which must be properly handled and disposed, and isolated from people's living area, so as to avoid environmental contamination and ensure that people can enjoy well-being of nuclear energy with safety. Radioactive wastes can be classified into three types, namely, solid wastes, liquid wastes and gas wastes according to the nature of state. Except that the radioactive wastes contain radionuclides, the radioactive wastes are similar to common industrial wastes, so the treatment technology of the radioactive wastes is similar to that of common wastes, but the radionuclides in the wastes need to be separated and intercepted at high treatment efficiency, and stabilized to prevent its release into the environment, so as to ensure that the ecological environment from being polluted. The treatment principle of radioactive liquid wastes includes separating and removing the radionuclides from the overall liquid waste to make the overall liquid waste meet environmental emission standards, and subjecting the overall liquid waste to volume concentration to facilitate subsequent fixing treatment and final disposal operations.
The fission product 137Cs in the radioactive liquid wastes generated in nuclear power plants has a half life period of moderately toxic radioactive nuclides, but the fission yield of 137Cs in the mass distribution curve of the fission product is up to 6.14%, which is the maximum content in the total radioactivity of the fission products of the used nuclear fuel after cooling, so 137Cs is the main radionuclide in the radioactive liquid wastes of nuclear power plants. Additionally, the liquid wastes generated in operation, during the decommissioning and the decontamination process of nuclear power plants and the laboratory wastes all contain 137Cs radionuclide. 137Cs has the following main properties: (1) having a long half life period, (2) having energy of γ ray, (3) belonging to Group IA of the sixth cycle in the periodic table of elements, and being easy to lose electrons to form a stable monovalent cation. However, most of the compounds in the environment are monovalent, so the stable monovalent cation of 137Cs can easily enter the environment, thus being harmful to humans. Therefore, it is required to remove the 137Cs radionuclide in the liquid wastes to reduce the volume and the mass, and to immobilize the 137Cs radionuclide for storage and final disposal.
The currently commonly used methods for treating radioactive liquid waste containing 137Cs radionuclides are mainly classified into the types below:
(1) Evaporation concentration method: After being treated by the evaporation concentration method, the concentrate of the radioactive liquid waste can be finally disposed after fixing, and the distillate can be treated by an ion exchange resin and then discharged. However, this treatment method has the main disadvantage that the energy consumption is too high, and if most of the radioactive liquid wastes is weak acids and salts, for example, when the specific activity of the 137Cs radionuclide is about 107 to 109 Bq/L, and the salt NaCl content in the concentrate approximately is 400-500 g/L, severe corrosion of evaporation concentration equipment easily occurs. Therefore, for the evaporation concentration technology, high-salt content 137Cs radionuclide liquid wastes are difficult to be treated.
(2) Natural silicon aluminate treatment method: A natural silicon aluminate of Kaolin, rectorite and vermiculite having certain ion exchange capacity is filled in a column for stream washing treatment of separated radionuclide liquid wastes. However, for this type of natural silicon aluminates, and the ion exchange capacity and the selectivity for the 137Cs radionuclide are low, so a large amount of secondary solid wastes are generated.
(3) Zeolite treatment method: Common natural zeolite or artificial synthetic zeolite has a regular crystal structure, and can effectively adsorb radionuclides, and theoretically has good adsorption capacity for 137Cs radionuclide. However, when the radioactive liquid wastes contain other monovalent ions (such as K+ or Na+), zeolite cannot effectively adsorb the 137Cs radionuclide, so the zeolite treatment manner has low selectivity for the 137Cs radionuclide.
(4) Ferrocyanide treatment method: Ferrocyanide salts (such as Co, Ni, Zn and Cu) have extremely high selectivity for the 137Cs radionuclide, when the Na+ concentration is 5 mole/L, the selectivity coefficient for Cs may be up to 1,500,000 (Nuclear Science and Engineering, 137, 206-214, 2001). However, the ferrocyanide salt powder is extremely fine and cannot be effectively applied in columns for practical treatment of a large amount of liquid wastes. According to relevant researches of Marden, with silicon dioxide as a support, ferrocyanide salts are fixed on silicon dioxide, and solvent evaporation is repeated for multiple times. The method has the disadvantages that a large amount of organic solution is consumed, and the maximum load is merely 1.36 g-ferrocyanide salts/g-SiO2, thus having little practical application. According to the patent specification of CN100551519C, urea-formaldehyde condensed TiO2 is immersed in potassium ferrocyanide (0.7 M) and hydrochloric acid (1 M) to obtain a TiO2-potassium ferrocyanide adsorbent material. However, this material has a not high specific surface area (about 14 m2/g) and a high mechanical strength, and cannot be easily smashed, and cannot be easily fixed with the urea-formaldehyde condensed TiO2 material. Furthermore, Terada (Talanta 1970, 17, 955-963) and Konecny (Radioanal. Chem., 1973, 14, 255-266) both describe a method including first adsorbing potassium ferrocyanide in silicon dioxide, and then converting with transition metallic ions (such as Co, Ni, Zn and Cu) to form a ferrocyanide salt adsorbent. However, the conversion reaction is extremely slow, excessive metal ions need to be used, the conversion reaction is difficult to control, and the loss amount of potassium ferrocyanide in silicon dioxide during the reaction with ions.
In view of the above, among the methods for treating 137Cs radionuclide radioactive liquid wastes, an inorganic adsorbent (natural silicon aluminate, zeolite and ferrocyanide salts) are mainly used to adsorb the 137Cs radionuclide. However, the natural silicon aluminate and zeolite have low selectivity to the 137Cs radionuclide and low adsorption capacity. Ferrocyanide salts have higher selectivity to the 137Cs radionuclide and the adsorption capacity, compared with other inorganic adsorbents. However, after synthesis, ferrocyanide salts are mainly in the form of a powder and are easily dispersed in water, and most of persons of ordinary skill in field of radioactive liquid waste treatment adopts a column to treat liquid wastes, since ferrocyanide salts are in the form of a powder, pressure drop in the column is increased, resulting in excessively high number of times of stream washing and generation of excessive secondary wastes. Therefore, the effect of practical treatment of radioactive liquid wastes is not ideal.